High-pressure electric discharge lamp having electrolysis-preventing means

ABSTRACT

The arc tube of an arc discharge lamp has two main electrodes and a starter electrode adjacent one of them, the starter electrode being electrically connected to the adjacent electrode through a resistor external of the arc tube. The starter electrode is electrically connected to the other main electrode through a thermally operative switch and a second resistor. In normal operation, the switch opens after lamp ignition to isolate the starter electrode from the circuit of the other main electrode and to place the starter electrode at substantially the same potential as the adjacent main electrode.

Unite States Patent [72] Inventor John F. Waymouth Marblehead, Mass. [21] Appl. No. 872,269 [22] Filed Oct. 29, 1969 [45] Patented Nov. 9, 1971 [73] Assignee Sylvania Electric Products Inc.

[5 4] HIGH-PRESSURE ELECTRIC DISCHARGE LAMP HAVING ELECTROLYSIS-PREVENTING MEANS 1 Claim, 3 Drawing Figs.

[52] U.S.Cl

315/47, 313/198, 313/228, 315/59, 315/60, 315/73 [51] Int. Cl H01] 17/34 I50] Flcldo! Search 313/25, 26, 198, 228, 229; 315/46, 47,49, 59, 60, 73, 100, 104 |56l References Cited UNITED STATES PATENTS 3,226,597 12/1965 Green 315/60 2/1967 Fraser et al. 5/1969 Thouret et a1.

OTHER REFERENCES Kanoh et al., Toshiba High-Efficiency Halide Lamps Toshiba Review Vol. 23, N0. 9, Pgs. 1126- 1 129 1966 Primary ExaminerRoy Lake Assistant ExaminerE. R. LaRoche AttorneyrNorman .l. OMalley and James Theodosopoulos ABSTRACT: The are tube of an arc discharge lamp has two main electrodes and a starter electrode adjacent one of them, the starter electrode being electrically connected to the adjacent electrode through a resistor external of the arc tube. The starter electrode is electrically connected to the other PATENTEnuuv 9 l9?! 3.619.710

SHEET 1 0F 2 FIG.I

JOHN F WAYMOUTH INVENTOR BYyu wu 71 0M AGENT PATENTEDN 9 SHEET 2 0F 2 JOHN F. WAYMOUTH INVENTOR BY T AGENT HIGH-PRESSURE ELECTRIC DISCHARGE LAMP HAVING ELECTROLYSIS-PREVENTING MEANS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the field of high pressure arc discharge lamps and is especially applicable to such lamps having a metallic halide fill.

2. Description of the Prior Art High-pressure arc discharge lamps generally comprise an elongated arc tube containing an ionizable fill and having press seals at each end of the tube. Disposed within the arc tube are two main electrodes, one at each end. The electrodes are generally supported in the press seals and are usually connected to a thin molybdenum ribbon disposed within the press seal, the purpose of the ribbon being to prevent seal failures because of thermal expansion of the lead-in wire.

In order to facilitate starting of the arc discharge, that is, ionizing of the gas fill, a starter electrode is generally disposed in the arc tube, adjacent one of the main electrodes. Such an electrode is used because an arc can be ignited between the starter electrode and its adjacent electrode at a much lower starting voltage than is required to ignite an are between the two main electrodes. Once the arc has ignited, the ionizing gas decreases the resistance between the two main electrodes and an arc is formed therebetween. At this time, it becomes desirable to electrically remove the starter electrode from the circuit or, at least, to maintain it at the same potential as the adjacent electrode, for reasons to be presently shown.

During operation of the lamp, electrolysis between the starter electrode and adjacent electrode can occur at the press seal, if there is an electric potential therebetween. The electrolysis current consists mainly of alkali ion flow and thus is greater in an arc tube having a fill that includes an alkali than in one that does not. However, electrolysis can always be present, since the arc tube material, generally high-silica glass or quartz, usually contains minute quantities of alkali metals.

Electrolysis current usually produces harmful results only when the starter electrode is at a negative DC potential with respect to the adjacent electrode; it can deteriorate, to the point of failure, the molybdenum ribbon to which the starter electrode is connected. Thus, even when the lamp is energized by an AC voltage, there can be a DC component of potential, as due to an asymmetry of starter electrode-to-adjacent electrode voltage. Such an asymmetry results from the fact that the starter electrode collects electrons more readily than ions. This leads to a DC potential, with starter electrode negative, that results in electrolysis.

In the prior art, the means used to eliminate a potential therebetween was a temperature sensitive switch, such as a U- shaped strip of bimetal, which, upon heating thereof, shorted the starter electrode lead-in wire to the adjacent electrode lead-in wire. A short period of time, say, about 30 seconds, was all that was normally required for the switch to heat up sufficiently to deflect and short the wires. As long as the wires were shorted and, thus, were at the same potential, no electrolysis could occur between the two electrodes. However electrolysis could occur during the period of time required for the switch to close.

During operation of the lamp, prolonged exposure of the switch to the heat emanating from the arc tube could cause the bimetal to take a set in the stressed, or shorted position, with the result that the switch could require progressively longer time intervals to close. These longer time intervals also permitted electrolysis to occur for longer periods of time, with the result that the life of the lamp could be appreciably shortened by the effect thereof.

It is an object of this invention to provide an arc discharge lamp having improved starting means which substantially eliminates lamp failures due to electrolysis.

SUMMARY OF THE INVENTION A lamp in accordance with this invention has an arc tube having press seals at each end, an ionizable fill including mercury, metal halide and an inert gas in the arc tube, two opposing main electrodes, a starter electrode adjacent one of the main electrodes and metal ribbon connectors in the press seals. Each electrode is electrically connected to separate ribbon connectors which, in turn, are connected to lead-in wires leading externally of the arc tube. The lamp has means to electrically connect each main electrode to opposite sides of an AC power supply.

The starter electrode is electrically connected to the adjacent main electrode through a resistor. In addition, the starter electrode is electrically connected to the same side of the power supply as the opposite main electrode through a thermally operative switch and another resistor. Essentially, the switch and two resistors comprise a series circuit across the power supply with the main electrodes directly connected to opposite sides of the power supply and the starter electrode electrically connected to the circuit at a point between the resistors.

The thermally operative switch is closed at the nonnal ambient temperatures of the unignited lamp but, after lamp ignition, is heated sufficiently by the arc tube to open and remain open during normal lamp operation. Upon extinguishment of the arc and cooling of the lamp, the switch recloses.

The ohmic value of the resistor between the starter electrode and the adjacent main electrode must be high enough so that the voltage developed thereacross, upon application of a suitable supply voltage to the lamp, is sufficient to strike an are between the starter electrode and the adjacent main electrode. Once the arc has ignited, the ionizing gas within the arc tube decreases the resistance between the two main electrodes and an arc is formed therebetween, since the opposing main electrode is at a higher potential than the starter electrode.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view of a high-pressure arc discharge lamp in accordance with this invention.

FIGS. 2 and 3 are perspective views of the switch portion of the lamp showing the switch closed and open, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in the drawings an arc discharge lamp in accordance with this invention comprises an outer vitreous envelope or jacket 2 of generally tubular form having a central bulbous portion 3. Jacket 3 is provided at its end with a sealed reentrant stem 4 through which extend relatively stiff lead-in wires 5 and 6 connected at their outer ends to the electrical contacts of the usual screw type base 7. Centrally disposed within jacket 2 is arc tube 8 which is supported at its lower end by metal frame 9 which, in turn, is welded to lead-in wire 6. Attached to the upper end of arc tube 8 is metal frame 10, part of which frictionally engages the upper tubular portion of jacket 2 and stabilizes the position of arc tube 8.

Are tube 8 is made of quartz, a high-silica glass, although other types of glass having comparable or higher softening temperatures may be used. Sealed in arc tube 8, at the opposite ends thereof, are main discharge electrodes 11 and 12 which are supported on lead-in wires 13 and 14 respectively. Each main electrode comprises a core portion which may be a prolongation of wires 13 and 14 and may be prepared of a suitable electrode metal such as tungsten. The prolongations of wires 13 and 14 can be surrounded by tungsten wire helixes.

An auxiliary starting electrode 15, generally prepared of tungsten, is provided at the lower end of arc tube 8 adjacent main electrode 11 and comprises an inwardly projecting end of another lead-in wire.

The ends of the lead-in wires are welded to molybdenum ribbon connectors which are completely embedded within the press seal ends of arc tube 8. Relatively short molybdenum wires 16 and 17 are welded to the ends of the molybdenum ribbon connectors and serve to convey current to electrodes 11 and 12 respectively.

Wire 16 is electrically connected through metal frame 9 to lead-in wire 6 by means of nickel strip 19 connected between wire 16 and frame 9.

Wire 17 is electrically connected to lead-in wire by means of wires 20 and 21. Wire 20 is welded directly to wire 17 and wire 21 is a long thin wire extending from the upper portion of jacket 3 to lead-in wire 5.

Starter electrode is connected to a rigid wire 18 which is supported in the press seal of arc tube 8. Wire 18 is electrically connected to lead-in wire 6 through current-limiting resistor 22, resistor 22 being connected between wire 18 and frame 9.

Disposed between lead-in wires 5 and 6 and embedded in stem 4 is a blank stud 23 the purpose of which is to support the lower end of bimetallic switch 24. The upper unsupported end of switch 24 is in contact with wire 18 at normal ambient lamp temperatures, as shown in FIG. 2. At normal operating temperature, switch 24 is heated sufficiently by are tube 8 to deflect away from wire 18, thereby breaking electrical contact therewith, as shown in FIG. 3. Switch 24 is mounted so as to have an unrestricted deflection path at normal operating temperatures in order to prevent imposition of contact stress on switch 24 at elevated temperatures. Upon extinguishment of the lamp, and cooling thereof, switch 24 returns to its original contact position with wire 18.

Electrically connected between stud 23 and lead-in wire 5 is resistor 25, the purpose of which is to electrically connect starter electrode 15 to lead-in wire 5, when switch 24 is closed. Resistors 22 and 25 comprise a series circuit between lead-in wires 5 and 6 and, when an external voltage is applied to lead-in wires 5 and 6, switch 24 being closed, the starting voltage applied between electrodes 11 and 15 equals the voltage drop across resistor 22. Since the total voltage drop across resistors 22 and 25 equals the total externally applied voltage, v

the voltage drop across resistor 22, that is, the lamp-starting voltage, equals that proportion of the applied voltage that the ohmic resistance of resistor 22 bears to the total resistance of resistors 22 and 25. In general, the starting voltage should be at least 50 percent of the applied voltage, which means that the resistance of resistor 22 should be at least equal to, or

greater than, that of resistor 25.

Although resistor 22 is electrically connected between main electrode 11 and starter electrode 15, the resistance of resistor 22 is small compared to the impedance of the DC generator constituted by the gap between electrodes 11 and 15 in the are. For example, in one case, resistor 22 had a resistance of 40,000 ohms and the impedance of the gap was about 1 megohm. Thus, resistor 22 is almost as effective as a dead short in maintaining electrodes 11 and 15 at the same DC potential and is effective in substantially eliminating electrolysis current therebetween.

The structure and operation of switch 24 results in improved reliability and permits great flexibility in the choice of materials that may be employed in its construction. Since switch 24 can be straight, it is less likely to age than bimetal switches that are curved or U-shaped. If it is desired to use higher temperature bimetals of lower intrinsic sensitivity, in order to reduce aging efi'ects even further, the switch may simply be lengthened to provide the desired rapid opening.

lclaim:

1. An electric discharge device comprising: an arc tube disposed within an outer jacket; an ionizable filling comprising mercury, metal halide and an inert gas within the arc tube; a first and second main electrode, and a starter electrode adjacent the first electrode, disposed within the arc tube; a first resistor electrically connected between the first and starter electrodes; a combination, comprising a second resistor in series with a thermostatic switch, electrically connected between the second and starter electrodes; a reentrant stem sealed at the base of the jacket and an internally extending stud sealed in the stem, the switch being attached to the stud and longitudinally disposed between said stud and said arc tube; the switch comprising a long straight strip of bimetal, the contact end of the bimetal being in electrical engagement, at normal ambient temperatures of the device, with a rigid wire sealed in the arc tube, the rigid wire being electrically connected to the starter electrode, the resistance of the first resistor being at least equal to the resistance of the second resistor but low enough in relation to the impedance of the DC generator constituted by the gap between the first electrode and the starter electrode as to be almost efiectively a dead short therebetween.

=9 III =1 

1. An electric discharge device comprising: an arc tube disposed within an outer jacket; an ionizable filling comprising mercury, metal halide and an inert gas within the arc tube; a first and second main electrode, and a starter electrode adjacent the first electrode, disposed within the arc tube; a first resistor electrically connected between the first and starter electrodes; a combination, comprising a second resistor in series with a thermostatic switch, electrically connected between the second and starter electrodes; a reentrant stem sealed at the base of the jacket and an internally extending stud sealed in the stem, the switch being attached to the stud and longitudinally disposed between said stud and said arc tube; the switch comprising a long straight strip of bimetal, the contact end of the bimetal being in electrical engagement, at normal ambient temperatures of the device, with a rigid wire sealed in the arc tube, the rigid wire being electrically connected to the starter electrode, the resistance of the first resistor being at least equal to the resistance of the second resistor but low enough in relation to the impedance of the DC generator constituted by the gap between the first electrode and the starter electrode as to be almost effectively a dead short therebetween. 